Articulated vehicles, i.e. vehicles in which steering is achieved by articulation of pivotally interconnected frame sections, generally have found a favorable reception in the market place for some time, including use in agricultural tractors. Open-center hydrostatic steering units have also been in common use for steering those agricultural tractors with front wheel steering, commonly referred to as Ackermann type steering. The Ackermann type steering, which is also used extensively on automobiles, can be designed, through proper angling of the kingpin, to provide a self-centering action, and drift, i.e. a slow wandering of the vehicle from the intended path, is generally not a problem. However, with articulated farm tractors, the Ackermann steering attributes are not available and the equivalent thereof is difficult to achieve with an inclined pin connection between the frame sections because the steering action itself can introduce stresses into the frame and can, to some extent, shift the distribution of vehicle weight between the front and rear axles. A straight or vertical pin connection is therefore desirable, as is the use of a hydrostatic steering unit because the latter permits limited, but usually effective, steering of a slowly moving vehicle with the engine dead.
The problem with using hydrostatic steering units, particularly the open-center type, is the slight leakage which is present in the hand pump of such units. When used on agricultural tractors the problem is even more pronounced because the draft load imposed on the tractor does not always have a line of action in the central plane of the tractor. This off-set loading tends to cause the tractor to drift, and with no centering action to off set it and with small but continuous leakage in the hand pump of the hydrostatic unit, the operator of the tractor must continually make directional corrections with the steering wheel.
It is, therefore, an object of the present invention to provide a hydrostatic steering arrangement for an articulated vehicle in which drift of the vehicle is minimized.
It is also an object of this invention to provide a hydrostatic steering arrangement for an articulated farm tractor in which steering corrections in order to compensate for offset draft loads imposed on such a tractor are reduced.
It is also an object of this invention to provide a simple and relatively inexpensive means which will permit use in an articulated farm tractor of a hydrostatic steering unit intended for use in a farm tractor with Ackermann type steering.
These and other objects, and many of the attendant advantages there of, will become more readily apparent upon a perusal of the following description and the accompanying drawing, wherein the sole FIGURE is a schematic of a hydrostatic steering system incorporating the present invention.